The purpose of this project is to gain a better understanding of some of the nonpharmacological environmental factors which contribute to drug effects in humans. The basic methodology employed involves studying drug effects on naturalistic human social behavior, and on subjective reports of mood under controlled laboratory conditions. Drug-produced changes in naturalistic human behaviors such as interpersonal social behavior, which have been demonstrated in previous research, may be an important component of the desirable or reinforcing effects of drugs of abuse. Initial studies will extend the analysis of acute drug effects on interpersonal behavior to several additional drugs which are commonly used and sometimes abused by humans, diazepam, marihuana, diethylpropion and oral methadone, as well as to fenfluramine, an anorectic drug which appears to lack abuse liability. Each of these drugs will be studied across a range of doses under comparable experimental conditions. These studies will contribute to an existing data base which characterizes effects of drugs of abuse on interpersonal social behavior, and will allow a better understanding of the contribution of these behavioral drug effects to the choice of abused drugs by humans. Subsequent studies will examine the modification of behavioral and subjective drug effects in humans under conditions which both add and subtract environmental interpersonal stimuli. One set of studies will determine whether facilitation of the behavioral and subjective effects of abused drugs occurs when these are ingested in the presence of other drugged group members. A second set of studies will examine the generality of drug effects on vocal behavior which occurs in an interpersonal and a noninterpersonal context. These studies will determine the extent to which certain behavioral and subjective drug effects may be modified by the presence and absence of interpersonal social stimuli. Overall, these studies will contribute to our understanding of drug effects on interpersonal behavior as well as the ability of environmental context to modify behavioral and subjective effects of drugs of abuse.